A commercially suitable Soft-X-Ray (or EUV) lithography facility will require an intense soft x-ray/EUV light source that can radiate within a specific wavelength region of approximately 11 to 14 nm in the EUV part of the electromagnetic spectrum. This region is determined by the wavelength range over which high reflectivity multilayer coatings exist. The multilayer coatings can be used to manufacture mirrors which can be integrated into EUVL stepper machines. Specifically, these coatings are either Mo:Be multilayer reflective coatings (consisting of alternate ultrathin layers of molybdenum and beryllium) which provide high reflectivity between 11.2 and 12.4 nm, or Mo:Si multilayer reflective coatings (consisting of alternate ultrathin layers of molybdenum and silicon) which provide high reflectivity between 12.4 nm and 14 nm. Thus any intense EUV source emitting in the wavelength range of 11-14 nm may be applicable to lithography. Two proposed EUV sources are synchrotrons which generate synchrotron radiation and soft-x-ray emitting laser-produced plasmas (LPP's). Synchrotron sources have the following drawbacks: the synchrotron and synchrotron support facilities cost up to $100 million or more; together they occupy a space of approximately 1,000,000 cubic feet. Such a volume is incompatible with a typical microlithography fabrication line. Laser produced plasmas that have the necessary wavelength and flux for a microlithography system require a high power laser to be focused onto a target material such that sufficient plasma density can be produced to efficiently absorb the incident laser radiation. Laser produced plasmas have the following drawback: if a solid target material is used, the interaction of the focused laser beam with the target produces an abundant quantity of debris which are ejected from the laser focal region in the form of atoms, ions, and particulates. Such ejecta can accumulate on and thereby damage the optics that are used in collecting the light emitted from the plasma. The use of volatile target materials in LPP sources has been successful in overcoming the debris problem. A volatile target material is simply a material which is unstable to evaporation in a room temperature vacuum, examples of these are liquified or solidified gases such as oxygen or xenon, and also liquids such as water. For these materials, any bulk mass not directly vaporized by the laser pulse will evaporate and will be subsequently pumped away. Thus the excess target material does not collect or condense on the optics.
Although such laser-produced plasma sources have been developed for EUVL using oxygen and xenon as radiating species, there still exist two prohibitive drawbacks for which no realistic scenarios of significant improvement have been proposed. First, the total electrical efficiency of such sources is of the order of only 0.005-0.025%. This results from considering the multiplicative combination of the laser efficiency, which is of the order of 1-5%, and the conversion efficiency of laser light to useful EUV radiation (within the reflectivity bandwidth of a multilayer-coated reflecting mirror) of approximately 0.5%. Second, the cost of a laser that would necessarily operate at repetition rates of over 1 kHz would be a minimum of several million dollars.
To overcome the unique problems specific to the synchrotron sources and to the LPP sources we have invented a compact electrically produced intense capillary discharge plasma source which could be incorporated into an EUV lithography machine. Compared to synchrotrons and LPP's this source would be significantly more efficient, compact, and of lower cost (both to manufacture and to operate). We envision that one of these sources (along with all the necessary support equipment) would occupy the space of less than 10 cubic feet and would cost less than $ 100,000. One such embodiment of the proposed capillary discharge source was first described in U.S. Pat. No. 5,499,282 by William T. Silfvast issued on Mar. 12, 1996. That particular proposed source would operate in a lithium vapor electrically excited to within specific ranges of plasma electron temperatures (10-20 eV) and electron densities (10.sup.16 to 10.sup.21 cm.sup.31 ) which are required for optimally operating a lithium vapor discharge lamp at 13.5 nm. That same patent also proposed soft-x-ray lamps at wavelength of 7.6, 4.86, and 3.38 nm in beryllium, boron, and carbon plasmas. These wavelengths, however, are not within the range of wavelengths required for EUV lithography Although that patent described the general features of these lamps, it did not give the specific discharge current operating range that would minimize bore erosion and the emission of debris from the lithium lamp, or the appropriate range of bore sizes for operating such a lamp. That patent did not mention the use of other materials, such as atomic or molecular gases that could be successfully operated in the lamp configurations described in that patent; it naturally follows that neither could it have mentioned what are the preferred operating pressure ranges of those gases that would be suitable for EUV lithography.